Many link layer protocols support a reliable data transport by performing retransmissions for failed transmissions. Failed transmissions are reported by feedback messages, such as acknowledgement (ACK) and non-acknowledgement (NACK) messages according to Automatic Repeat Request (ARQ) protocols. ARQ mechanisms are, in particular, important for wireless transport media, but are also applied to wired links. Examples for ARQ mechanisms operating over wireless channels include the Radio Link Control (RLC) protocols for General Packet Radio Service (GPRS) and Wideband Code Division Multiple Access (WCDMA), as well as the Hybrid ARQ (HARQ) protocol in Medium Access Control high-speed (MAC-hs) for High-Speed Downlink Packet Access (HSDPA). A problem with such protocols is that they may not provide fast and reliable feedback and efficient use of radio resources.
Some prior art protocols use a simple and fast ACK/NACK concept which indicates whether a data frame has been successfully received. Such protocols provide no sequence numbers in the feedback, but instead the transmitter and receiver implicitly relate the feedback to a particular transmission by exploiting a fixed timing relationship. This is often referred to as synchronous feedback. The advantage of such an approach is that these short signals can be sent often as the transmission resource consumption is relatively low. Achieving coding gains, however, is limited, or impossible, if each ACK or NACK is a single bit. Thus, there is the risk of misinterpretation of such single bit at the receiver. Fading dips further increase the error probability, and achieving a very low error rate can consume a lot of resources to cover “worst-case” dips. Thus, such signalling is also costly if very low error ratios are required, because that can only be achieved by increasing the transmission power or by repeating the information. An update, or retransmission, of each feedback message is not possible, however, as it needs to be time-aligned to the transmission of the corresponding data.
Another class of protocols uses feedback, or control, units (sometimes called Status Messages). Such mechanisms are often applied for window-based ARQ protocols. Feedback units may explicitly include sequence numbers and a checksum and, therefore, the reliability of feedback messages can be confirmed. Incorrectly received feedback is not used, but dropped at the data sender. Retransmissions or transmissions of up-dates of the feedback are used to ensure that the feedback is correctly received. It should be noted that such feedback units do not require any time-alignment to the corresponding data units due to the sequence numbering of data units and the reference to those in the feedback units. These types of feedback mechanisms have the advantage of being very reliable; however, they are typically much slower compared to synchronous ACK/NACK feedback mechanisms.
Therefore, what is needed in the art are integrated retransmission protocols that achieve the efficiencies of conventional ACK/NACK protocols, while simultaneously realizing the reliability of explicit feedback messages. Preferably, such integrated retransmission protocols can be implemented in a single protocol entity and are based on the same protocol data units, protocol state and logic.